In recent years, since a demand to reduce carbon dioxide (CO2) emissions is increasing for global environmental concern, lean-burn operation has been aimed to improve fuel efficiency of an internal combustion engine for a vehicle. Meanwhile, nitrogen oxide (NOx) cannot be substantially reduced by conventional three-way catalysts because exhaust gas from lean burn engines using gasoline, direct-injection engines and diesel engines contains a lot of oxygen because of lean combustion. Now, diverse technological developments have been enhanced to efficiently purify the exhaust gas from engines, especially from diesel engines.
One of the effective ways for such purification is to use a NOx trap catalyst. The NOx trap catalyst traps nitrogen oxide in the exhaust gas when an air-fuel ratio of the incoming exhaust gas is under lean condition, and releases and reduces the trapped nitrogen oxide when the air-fuel ratio of the incoming exhaust gas is under theoretical (stoichiometric) or rich condition. Thus, the trapped NOx is released and reduced by making the air-fuel ratio of the exhaust gas theoretical or rich before exceeding the allowable trapped amount of nitrogen oxide. Then, nitrogen oxide is reduced by increased reductants (hydrogen (H2), carbon monoxide (CO), hydrocarbons (HC)). However, it is well known that excessive reductants, especially excessive hydrocarbons, can cause environmental deterioration if they are discharged without being used for reduction of nitrogen oxide. Furthermore, it is undesirable that the air-fuel ratio of the exhaust gas is rapidly shifted to theoretical or rich to increase the reductants because it results in worsening driving performances and lowering fuel efficiency.
Accordingly, it has been attempted that hydrogen is especially used as a more effective reductant for reducing nitrogen oxide. Also, it has been suggested a catalyst producing hydrogen by steam reforming (see Patent Citation 1).
Patent Citation 1: Japanese Patent No. 3741303
However, since the steam reforming reaction is an endothermic reaction, heat supply is needed to get a sufficient reaction speed, i.e. it is required to bring catalysts under high temperature condition. Therefore, this is hardly a practical solution in order to bring a sufficient effect of nitrogen oxide purification under actual driving conditions. In addition, it results in environmental deterioration by making the air-fuel ratio of the exhaust richer so as to enhance a reduction ratio of nitrogen oxide because of the increase of discharge amounts of unreacted hydrocarbons. Therefore, it is necessary to increase catalysts more to remove hydrocarbons. Thus, it has been suggested to provide a HC adsorbing catalyst in addition to the NOx trap catalyst (see Patent Citation 2).
Patent Citation 2: Japanese Patent Unexamined Publication No. 2003-206785
Another method has been suggested that a hydrocarbon adsorbent and a steam reforming catalyst are combined so as to purify hydrocarbons by using water under less oxygen condition (e.g. see Patent Citation 3).
Patent Citation 3: Japanese Patent Unexamined Publication No. 2002-282697
Patent Citation 3 shows a good result in the model experiment under a low space velocity (SV) using poorly-adsorbing methane. Also, experiments using engines have been performed, in which air-fuel ratios are around theoretical (A/F=13.5-15.5) with a three-way catalyst in addition to the hydrocarbon adsorbent and the steam reforming catalyst. However, an ability of release and purification of hydrocarbons is 40%, which is relatively low. In addition, an ability of reduction of nitrogen oxide is unclear. An effect of the combination of the NOx trap catalyst and the HC adsorbing catalyst is also obscure.